Biaxially oriented polypropylene films (hereinafter referred to as OPP films) are typical of polyolefin films. Because of their excellent transparency and gloss, OPP films are used as packaging materials in a wide range of fields. OPP films further have high rigidity, and hence have excellent printability and excellent suitability for bag-forming and filling operations. However, when the conventional OPP films are used in bag-forming by heat sealing/cutting, the heat-sealed parts have a low sealing strength and large fluctuations in sealing strength. Conventional OPP films therefore are disadvantageous in, for example, that bag breakage is apt to occur during filling and sealing, and also apt to occur due to vibration during the transportation or handling of the packages.
Under these circumstances, a multilayered OPP film comprising an OPP film having a heat-sealable layer has been proposed (see JP-B-7-17043). (The term "JP-B" as used herein means an "examined Japanese patent publication".) However, the proposed film having a low-melting heat-sealable layer also has large fluctuations in heat-sealing strength and are apt to develop pinholes in parts having a low heat-sealing strength. Thus, the proposed film has a problem that it is unsuitable for use in food packaging for which airtightness is required.
In addition, an OPP film for heat sealing, employing an ethylene/propylene copolymer or an ethylene/propylene/butene copolymer has been proposed (see JP-A-7-241906). (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) However, the OPP films employing either of these copolymers are both inferior in stiffness as compared to conventional OPP films and hence have a problem that the suitability thereof for bag-forming and filling operations is reduced.